Among the most important scientific challenges facing society today is finding a way to meet the energy needs of the world's growing population via an environmentally sustainable paradigm. The sun represents the most abundant potential source of pollution-free energy on earth. Considerable research effort on photovoltaic materials and devices has led to much progress in the last 20 years. However, energy from current photovoltaic technologies is too expensive compared with that from fossil fuels. Novel materials and devices that could potentially revolutionize solar energy conversion technologies, making them cost-competitive with fossil fuels, are needed.
A solar cell (or photovoltaic cell) is a semiconductor device that directly converts absorbed sunlight (photons) into electricity. Incident photons in a semiconductor create excitons (bound electron-hole pairs) whose subsequent dynamics, relaxation, and dissociation are crucial to the photoconversion process. Equally important to the overall efficiency of the photon-to-electricity conversion is the nature of the charge carrier transport to collecting electrodes after exciton dissociation into free charge carriers.
Light absorption in organic/polymer semiconductors creates Frenkel excitons with large binding energies (˜0.4-1.0 eV) and small diffusion lengths (5-20 nm). Consequently, efficient photogeneration of free charge carriers in organic photovoltaics (OPVs) requires dissociation of excitons at a heterojunction with another material having highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energy level offsets suitable for exciton dissociation. Bulk heterojunction (BHJ) organic photovoltaic (OPV) cells, consisting of a binary blend or composite of a donor polymer and an acceptor material, address the problem of small exciton diffusion lengths (Ld=5-20 nm) in current organic/polymer semiconductors.
Extensive studies of such BHJ-OPV cells have focused largely on blends or nanocomposites of donor polymer with acceptor materials based on fullerenes. BHJ-OPV cells based on [60]- and [70]-fullerene derivatives (PCBMs) and donor polymers currently have high power conversion efficiencies achieved by optimization of factors such as molecular engineering of the donor polymer, blend composition, processing conditions, various annealing protocols, and use of processing additives. However, further advances in improving the efficiencies of polymer solar conversion systems to commercially useful levels (>15-18%) require major innovations in acceptor and donor materials and optimization of device architectures at the molecular- and nano-scales. In addition, a better fundamental understanding of the photoconversion processes, charge transport, and charge collection in BHJ solar cells is critical towards achieving the theoretical device conversion efficiency.
PCBM fullerene derivatives such as [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM), [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM), and other fullerenes have attributes which make them successful as acceptors in OPVs. These attributes include: (i) the existence of low lying excited states in the monoanions, which leads to substantial enhancement in charge separation rates without affecting the charge recombination rates; (ii) the large π-conjugated molecular structure which supports efficient electronic delocalization and polaron formation; (iii) the rigid molecular architecture and high molecular diffusion that facilitate facile aggregation into a phase-separated nanoscale morphology for efficient charge separation and transport; and (iv) the three-dimensional (3D) spherical structure, which results in a large decrease in Coulomb barrier for charge separation due to enhanced entropic effects and enables isotropic charge transport. The attributes can help guide the design of highly efficient non-fullerene electron acceptors for OPVs.
As discussed above, fullerene-based electron acceptors have provided the foundation for advances in fundamental understanding of charge photogeneration and practical developments in organic photovoltaics (OPVs) in the last 20 years. While donor polymers in OPVs have been successfully optimized in recent years, as shown by the steady increase in power conversion efficiency (PCE) of single-junction OPV cells from under 3% to current 7-9% as the donor polymer has changed from poly(phenylene vinylene) derivatives to poly(3-hexylthiophene) to narrow band gap copolymers, non-fullerene electron acceptors reported so far have shown significantly inferior electron accepting properties, resulting in bulk heterojunction (BHJ) solar cells with low PCEs (<3%). Nevertheless, the prospects of enabling new pathways to OPVs while overcoming the small photovoltage, high cost, and other limitations of fullerene-based OPVs motivate efforts to discover alternative organic electron acceptors.
In contrast to PCBMs and other fullerene acceptors, semiconductors (e.g., oligomeric semiconductors) that incorporate multiple chromophores into one molecule combine the advantages of small molecules (e.g., ease of synthesis, purification and no batch-to-batch variation in quality) and the favorable properties of macromolecules (e.g., large molar mass, good solution processability, good mechanical and physical properties, good film-forming properties, etc.) and can be promising electron acceptor candidates. Thus, design and synthesis of electron acceptors that possess similar electronic structures and charge generation/transport behaviors as fullerenes, but that overcome the limitations of fullerenes, can be an efficient approach to high performance electron acceptors for OPVs. The present disclosure seeks to fulfill these needs and provides further related advantages.